1. Field of the Invention
The present invention relates to a slot antenna and, more particularly, to a dual operational frequency slot antenna for receiving and transmitting at dual operational frequencies using a circularly polarized wave.
2. Description of the Related Art
Portable communication systems often use circularly polarized radiation. Several applications further require a dual-band circularly polarized operation. For example, a dual-band right-hand circularly polarized antenna for GPS applications operates at both 1575.42 and 1227.60 MHz. For other potential applications, circularly polarized antenna is suitable to be used in a reader to detect tags in an RFID (Radio Frequency IDentification) system because the tags may not be polarized in a fixed direction. The operating frequency for RFID systems can be in a UHF (860–930 MHz) or in a microwave (2.45 GHz) band. For dual-band operation, the reader's antenna can be designed to transmit/receive two circularly polarized radiation signals at two different frequencies. The type of the antenna can best be a microstrip or a slot antenna, which helps to reduce the overall size of the conventional antenna. A lot of microstrip antenna technologies are existed, such as U.S. Pat. No. 6,509,873, entitled “Circularly polarized wideband and traveling-wave microstrip antenna”, or U.S. Pat. No. 6,522,302, entitled “Circularly-polarized antennas”. However, the above-mentioned technologies only operate at a single frequency.
Therefore, it is desirable to provide a dual operational frequency slot antenna to mitigate and/or obviate the aforementioned problems.